It is often beneficial to detect polynuclear aromatic hydrocarbons (PAHs) present within oilfield fluids, and fluids derived from processing oilfield fluids during oil refining and petrochemical processes. Polynuclear aromatic hydrocarbons (PAHs), also known as polycyclic aromatic hydrocarbons or poly-aromatic hydrocarbons, occur in oil, coal, and tar deposits, and are typically found within oilfield fluids and as a result of refinery and/or petrochemical processing of such oilfield fluids. PAHs are hydrocarbon structures with fused aromatic rings and do not typically contain heteroatoms or carry substituents. Although PAHs are aromatic compounds, the degree of aromaticity may be different for each ring segment. A PAH is typically characterized by the resonance structure with the most disjoint aromatic n-sextets—i.e. benzene-like moieties.
Drilling fluids are used in operations to drill boreholes into the earth; ‘drilling fluid’ is typically synonymous with ‘drilling mud’. One classification of drilling fluid is based on the composition of the fluid or mud. For example, drilling fluids include, but are not necessarily limited to, water-based fluids, brine-based fluids, oil-based fluids and synthetic-based fluids, which are synthetically produced rather than refined from naturally-occurring materials.
Production fluid is the fluid that flows from a formation to the surface of an oil well. These fluids may include oil, gas, water, as well as any contaminants (e.g. H2S, asphaltenes, etc.) therein. The consistency and composition of the production fluid may vary. Refinery and petrochemical fluids are results of processing production fluids. These processing methods may include distillation, delayed coking, hydrocracking, visbreaking, steam cracking of gas and distillate range products.
Hydrocracking is a catalytic cracking process using an elevated partial pressure of hydrogen gas to purify the hydrocarbon stream, e.g. polynuclear aromatic hydrocarbons, from sulfur and nitrogen hetero-atom that may have byproducts, such as napthenes and alkanes. Thermal cracking is a similar process where hydrocarbons, such as crude oil, are subjected to high heat and temperature to break the molecular bonds and reduce the molecular weight of the substance being cracked. The usable components are extracted, known as fractions, and released during the cracking process. These are two types of cracking methods used in the petroleum industry to process crude oil and/or other petroleum products for commercial use.
PAHs have very unique absorbance bands for each ring structure with respect to varying wavelengths of light. For a set of isomers, each isomer may have a different absorbance spectrum, which is useful in the identification of PAHs. PAHs may also be fluorescent when the molecules absorb light, i.e. emitting characteristic wavelengths of light when they are excited. Current examples of detecting PAHs within their respective materials may include gas chromatography-mass spectrometry, liquid chromatography with ultraviolet-visible, fluorescence spectroscopic methods, and using rapid test PAH indicator strips.
Several types of PAHs may be found within asphaltenes, coke, coke precursors, oil resins, carboids, and combinations thereof. Naphthalene is the simplest of the PAHs and has two coplanar six-membered rings that share an edge. Some do not consider naphthalene to be a true PAH, but it shall be defined as a PAH for purposes related to this application. Perylene has the chemical formula C20H12 with a structure of two naphthalene molecules connected by a carbon-carbon bond at the 1 and 8 positions on both molecules. All of the carbon atoms in perylene are sp2 hybridized. Coronene (also known as superbenzene) has six peri-fused benzene rings with a chemical formula of C24H12. Chrysene has four fused benzene rings and has the molecular formula C18H12. Chrysene may be a natural constituent or derived from coal tar, creosote, coal, crude oil, and plant material. Anthracene comprises three fused benzene rings with a chemical formula of C14H10.
Asphaltenes are most commonly defined as that portion of crude oil, which is insoluble in heptane. Asphaltenes exist in the form of colloidal dispersions stabilized by other components in the crude oil. They are the most polar fraction of crude oil, and often will precipitate upon pressure, temperature, and compositional changes in the oil resulting from blending or other mechanical or physicochemical processing. Asphaltene precipitation occurs in pipelines, separators, and other equipment. Once deposited, asphaltenes present numerous problems for crude oil producers. For example, asphaltene deposits can plug downhole tubulars, well-bores, choke off pipes and interfere with the functioning of separator equipment. The asphaltene deposits may also precipitate within a fluid and foul refining and/or petrochemical processes of such fluids. In addition to carbon and hydrogen in the composition, asphaltenes also may contain nitrogen, oxygen and sulfur species. Typical asphaltenes are known to have some solubilities in the oilfield fluid itself or in certain solvents like carbon disulfide, but are insoluble in solvents like light naphthas.
Coke is typically defined as a toluene, and/or an insoluble organic portion of crude oil, distillation residua, or residua from thermal/catalytic conversion processes, such as including but not limited to visbreaker tar or LC finer/H oil residuum. Coke may have PAHs dispersed therein with a ring structure of about 4 to about 5 or more condensed aromatic rings. Coke may be polymerized to a molecular weight where it is no longer soluble in crude oil or residua.
Coke precursors are the fragments that make up the coke, which may also include PAHs. Coke precursors may form from thermal cracking, dealkylation and/or dehydrogenation processes commonly used for the breaking down of complex organic molecules. They are barely soluble in the crude oil and/or residual, but they tend to precipitate. Once they precipitate, the coke precursors tend to polymerize or conglomerate into coke.
Coke and/or coke precursors are typically formed during thermal cracking and/or distillation during in situ combustion, which is a method of generating fire inside a reservoir by injecting an oxygen gas, such as air. A special heater in the well ignites the oil in the reservoir and starts a fire. The heat generated from burning the heavy hydrocarbons in situ produces hydrocarbon cracking vaporization of light hydrocarbons and reservoir water in addition to the deposition of heavier hydrocarbons known as coke.
When the oilfield fluid from a subsurface formation comes into contact with a pipe, a valve, or other production equipment of a wellbore, or when there is a decrease in temperature, pressure, or change of other conditions, these PAHs may precipitate or separate out of the oilfield fluid. While any separation or precipitation is undesirable in and by itself, it is much worse when the precipitants accumulate and stick to the equipment in the wellbore. Any precipitants sticking to the wellbore surfaces may narrow pipes; and clog wellbore perforations, various flow valves, and other wellsite and downhole locations and may result in wellsite equipment failures. It may also slow down, reduce or even totally prevent the flow of oilfield fluids into the wellbore and/or out of the wellhead.
Similarly, undetected precipitations and accumulations of PAHs in a pipeline for transferring crude oil could result in loss of oil flow and/or equipment failure. Crude oil storage facilities could have maintenance or capacity problems if PAH precipitations remain undetected for an extended period of time. Precipitation and accumulation of PAHs may also occur during refining of production fluids within the heater when trying to separate the production fluids into fractions of different boiling points. Finally, the precipitation of PAHs may also occur during the refinery and petrochemical processing of lighter distillate refinery streams such as, but not limited to, gas, gasoline, gasoils, and the like by thermal and/or catalytic conversion units. These include Fluid Catalytic Cracking (FCC unit) and steam cracking/ethylene cracking units in petrochemical plants.
Thus, it would be desirable if methods could be devised to better detect the presence, amounts and/or properties of polynuclear aromatic compounds commonly found within the aforementioned fluids.